The present invention relates to a scanning optical system which can be employed in imaging devices such as a laser beam printer.
A conventionally known scanning optical system is provided with a laser source which emits a laser beam, a deflector such as a polygonal mirror, which deflects the laser beam emitted by the laser source to scan within a predetermined angular range, and a scanning lens such as an fxcex8 lens which converges the deflected beam on an imaging surface to form a beam spot. As the laser beam scans within the predetermined scanning range, the beam spot moves on the imaging surface to form a scanning line thereon.
In general, the scanning lens of the scanning optical system as above is not designed to compensate for a lateral chromatic aberration, since the laser beam used in an imaging device has a single wavelength. Therefore, if the wavelength of the laser beam varies with respect to the designed wavelength due to individual differences of the laser diodes and/or changes of ambient temperature, a length of the scanning line varies due to the lateral chromatic aberration of the scanning lens. In such a case, accuracy of the image forming procedure becomes worse.
In order to compensate for the lateral chromatic aberration of the scanning lens, a diffraction surface like a surface of a Fresnel lens may be formed on a lens surface. Examples of such a configuration are described in Japanese Patent publications Nos. HEI 10-68903 and HEI 10-197820.
The diffraction surface has a plurality of stepped portions. In the above-identified publications, the diffraction surface is formed on a refraction surface of a refractive lens included in the scanning lens. In such a constitution, necessary phase shift should be achieved when a laser beam passes the diffraction surface once. Therefore, levels between adjacent steps (i.e., boundaries between adjacent stepped zones) tend to be relatively large. Such a structure is difficult to form accurately, and, due to processing error in forming the boundaries between respective steps, diffraction efficiency of the diffraction surface is lowered.
It is therefore an object of the invention to provide am improved scanning optical system having a diffraction surface that compensates for lateral chromatic aberration, in which reduction of diffraction efficiency due to processing error is avoidable.
For the above object, according to the present invention, there is provided a scanning optical system, which is provided with a light source, a deflector that deflects a light beam emitted from the light source to scan in a main scanning direction, a scanning lens that converges the light beam deflected by the deflector on an imaging surface to form a spot which scans on the imaging surface in the main scanning direction, and a mirror arranged between the deflector and the imaging surface, the mirror bending an optical path of the laser beam between the deflector and the imaging surface. With this structure, the reflecting surface of the mirror has a diffraction surface to compensate for the lateral chromatic aberration generated by the scanning lens.
Since the diffraction surface is formed on a reflecting surface located between the deflector and the imaging surface, optical path difference generated by the diffraction surface is twice, and a difference of levels between adjacent diffraction profiles can be suppressed relatively small in comparison with a case where such a diffraction surface is formed on a refraction surface of the scanning lens. Therefore, the diffraction surface can easily be manufactured accurately, and high diffraction efficiency can be obtained.
Optionally, the diffraction surface includes a part of a plurality of annular zones.
In this case, the mirror may be a front surface mirror. The mirror may be formed by injection molding and have a gate portion for injection The gate portion is preferably positioned closely adjacent to a center of the annular zones. With this structure, the injecting process can be achieved without fail.
Alternatively, the mirror may be a back surface mirror.
Optionally, the diffraction surface is formed on a base curve which is convex at least in the main scanning direction.
By forming the base curve to be convex in the main scanning direction, a bow caused by the diffraction power can be cancelled by the power generated by the base curve.
Alternatively, the base curve may be a flat surface, and the scanning system may be configured such that the light beam emitted from the light source is incident on the deflector at a predetermined angle in an auxiliary scanning section. With this configuration, the bow can also be cancelled.
Optionally, the scanning optical system may further include a cylindrical lens arranged between the light source and the deflector, the cylindrical lens forming a line-shaped image, elongated in the main scanning direction, of the laser beam. The scanning lens may include an anamorphic lens having a relatively strong positive power in the auxiliary scanning direction, the anamorphic lens being located relatively close to the imaging surface, and the anamorphic lens is arranged to be displaced in the auxiliary scanning direction with respect to an optical axis of the other lenses included in the scanning lens.